The present invention generally relates to the preparation of semiconductor grade single crystal silicon used in the manufacture of electronic components. More particularly, the present invention relates to a novel seed crystal for pulling a single crystal silicon ingot, in accordance with the Czochralski method, wherein a reduced number of dislocations are produced when the seed crystal contacts the crystal melt. Additionally, this invention relates to a process for the Czochralski growth of silicon single crystals, which uses such a novel seed crystal. The reduction in the number of dislocations produced upon contacting the seed crystal with the crystal melt results in the growth of dislocation-free crystals of large diameter and heavy weight having necks of relatively large diameter and short length.
Silicon single crystal, which is the starting material for most processes in the fabrication of semiconductor electronic components, is commonly prepared by the Czochralski (“Cz”) method. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted. A seed crystal is brought into contact with the molten silicon and a single crystal is grown by slow extraction. As crystal growth is initiated, dislocations are generated in the crystal from the thermal shock of contacting the seed crystal with the melt. These dislocations are propagated throughout the growing crystal and multiplied unless they are eliminated in the neck region between the seed crystal and the main body of the crystal.
The conventional method of eliminating dislocations within silicon single crystal (known as the Dash neck method) involves growing a neck having a small diameter (e.g., 2 to 4 mm) at a high crystal pull rate (as high as 6 mm/min) to completely eliminate dislocations before initiating growth of the main body of crystal. Generally, dislocations can be eliminated in these small diameter necks after approximately 100 to about 125 mm of neck is grown. Once the dislocations have been eliminated, the diameter of the crystal is enlarged to form a crown or taper portion until the desired diameter of the cylindrical main body is reached. The cylindrical main body of the crystal is then grown to have an approximately constant diameter by controlling the pull rate and the melt temperature while compensating for the decreasing melt level.
The neck, which is the weakest part of the silicon single crystal, can fracture during crystal growth causing the body of crystal to drop into the crucible. Thus, conventional crystals having a Dash neck are typically grown to a weight of 100 kg or less to minimize stress on the neck. However, in recent years, progress in the semiconductor industry has created an ever-increasing demand for larger silicon wafers of a high quality. Particularly, more highly integrated semiconductor devices have resulted in increased chip areas and a demand for the production of silicon wafers having a diameter of 200 mm (8 inches) to 300 mm (12 inches) or more. This has resulted in the need for a more effective necking process which eliminates dislocations and prevents neck fractures while supporting the growth of silicon single crystals weighing up to 300 kg or more.
A general solution for preventing neck fractures in larger crystals is to increase the neck diameter. However, large diameter necks are generally undesirable as they require larger seed crystals, which produce a higher density of slip dislocations when contacted with the silicon melt. Thus, larger diameter neck portions require an increased neck length and more process time to effectively eliminate slip dislocations.
In order to minimize the generation of slip dislocations in a larger diameter Dash neck, Japanese laid-open application (Kokai) No. 4-104988 proposed a process using a seed crystal having a unique, conical portion at its apex. However, such a seed crystal is complicated and expensive to process. Because the crystal is unique to a particular crystal pull, the seed crystal must be changed between each crystal pull regardless of whether dislocation-free growth is successful. Changing the seed crystal requires extra process downtime, which adversely affects productivity. Furthermore, the reference describes a heater embedded in the seed crystal holder, which makes it more difficult to form a temperature gradient between the seed crystal and the neck portion such that the single crystal must be pulled at an extremely slow rate.
Another method to prevent propagation of dislocations to the single crystal is disclosed in Japanese Patent Laid-Open No. 9-235186 and is directed toward pre-heating the seed crystal before contacting the crystal melt by interrupting the downward movement of the crystal at a position immediately above the melt. After pre-heating, the seed crystal is gradually lowered into the melt at a decreasing speed. This method is problematic because even if the seed crystal is stationary above the melt for a long period, the temperature cannot be raised in proportion to the time elapsed due to the low thermal conductivities of argon gas and silicon.
In view of the forgoing, it can be seen that a need continues to exist for a process that enables large diameter ingots of substantial weight to be grown by means of a neck having a comparably large diameter but short length.